O2 was the top online ad spender in the UK in 2010

"According to figures compiled exclusively for Marketing by Nielsen, mobile services provider O2 was the biggest online display advertiser in the UK, despite reducing its budget by almost 30% year on year to £9.9m.
Although the figures do not include spend on keyword search or social media, they suggest that major FMCG firms still allocate only a small proportion of their budgets to paid-for online advertising.
Procter & Gamble, the UK's biggest advertiser, devotes just 1.3% of its spend to paid-for online display ads, while Unilever invests £2.6m, just 1.9% of its total media spend.
The COI posted the biggest year-on-year fall in the top 10, slashing its spend by 52% year on year to £4.6m. Conversely, Google increased its investment by 175% to £4m, dedicating 61% of its total media spend to online display ads."
Source:  Data from Nielsen and Marketing Magazine, reported by Marketing Magazine, 22nd March 2011
Note:  As the figures don't include search (which is about 70% of all UK online ad spend), they should be treated with caution.